Mass Extinctions Change the Rules of Evolution

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Mass Extinctions Change the Rules of Evolution

Historical species diversity among marine animals of Cambrian, Paleozoic and Modern origin.

A reinterpretation of the fossil record suggests a new answer to one of evolution's existential questions: whether global mass extinctions are just short-term diversions in life's preordained course, or send life careening down wholly new paths.

Some scientists have suggested the former. Rates of species diversification – the speed at which groups adapt and fill open ecological niches – seemed to predict what's flourished in the aftermath of past planetary cataclysms. But according to the calculations of Macquarie University paleobiologist John Alroy, that's just not the case.

"Mass extinction fundamentally changes the dynamics. It changes the composition of the biosphere forever. You can't simply predict the winners and losers from what groups have done before," he said.

Alroy was once a student of paleontologist Jack Sepkoski, who in the 1980s formalized the notion that Earth has experienced five mass extinctions in the 550 million years since life became durable enough to leave a fossil record. Graphs of taxonomic abundance depict lines rising steadily as life diversifies, plunging precipitously during each extinction, and rising again as life proliferates anew.

As the fossil record is patchy and long-term evolutionary principles still debated, paleobiologists have historically disagreed about what these extinctions mean. Some held that, in the absence of extinctions, species would diversify endlessly. The Tree of Life could sprout new branches forever. Others argued that each taxonomic group had limits; once it reached a certain size, each branch would stop growing.

Sepkoski's calculations put him on the limits side of this argument. He also proposed that, by looking at the rate at which each group produced new species, one could predict the winners and losers of each mass extinction's aftermath. Groups that diversified rapidly would flourish. Their destiny was already established.

"It's a clockmaker vision of evolution. Each group has fixed dynamics, and if there's an extinction, it just messes it up a bit," said Alroy. "That's what I'm challenging in this paper. There are limits, and that's why we don't have a trillion species. But those limits can change."

Alroy crunched marine fossil data in the Paleobiology Database, which gathers specimen records from nearly 100,000 fossil collections around the world. He used a statistical adjustment method designed to reduce the skewing influences of paleontological circumstance – the greater chances of finding young fossils rather than old, the ease of studying some types of rock rather than others.

The analysis, published September 2 in Science, produced what Alroy considers to be the most accurate reflection of extinction dynamics to date. And while his data supported the notion that each group's diversity eventually hits a limit, he didn't find Sepkoski's correlation between pre-mass-extinction diversity rates and post-extinction success. Each mass extinction event seemed to change the rules. Past didn't indicate future.

In an accompanying commentary, paleontologist Charles Marshall of the University of California, Berkeley noted that Alroy's statistical methods still need review by the paleobiology community. The Paleobiological Database, for all its thoroughness, might also be incomplete in as-yet-unappreciated ways. "There will be no immediate consensus on the details of the pattern of diversity," he wrote. But "the pieces are falling into place."

Enough pieces have come together for Alroy to speculate on his findings' implication for the future, given that Earth is now experiencing another mass extinction. Starting with extinctions of large land animals more than 50,000 years ago that continued as modern humans proliferated around the globe, and picking up pace in the Agricultural and Industrial ages, current extinction rates are far beyond levels capable of unraveling entire food webs in coming centuries. Ecologists estimate that between 50 and 90 percent of all species are doomed without profound changes in human resource use.

In the past, many evolutionary biologists thought life would eventually recover its present composition, said Alroy. In 100 million years or so, the same general creatures would again roam the Earth. "But that isn't in the data," he said.

Instead Alroy's analysis suggests that the future is inherently unpredictable, that what comes next can't be extrapolated from what is measured now, no more than a mid-Cretaceous observer could have guessed that a few tiny rodents would someday occupy every ecological niche then ruled by reptiles.

"The current mass extinction is not going to simply put things out of whack for a while, and then things will go back to where we started, or would have gone anyway," said Alroy. Mass extinction "changes the rules of evolution."

Images: 1) A fossil skull of Dunkleosteus, an apex predator fish that lived between 380 million and 360 million years ago, and had what is believed to be history's most powerful bite./Michael LaBarbera, courtesy of The Field Museum. 2) Graph of species diversity among marine animals of Cambrian, Paleozoic and Modern origin./Science.